1. Field of the Invention
The present invention relates to a cordless system telephone apparatus, and more particularly, to a telephone apparatus comprising a control unit connected to a public communication network, a plurality of base units connected to the control unit, and a plurality of handset units which respectively establish an audio link with one of the base units.
2. Description of the Prior Art
A cordless system telephone apparatus, wherein base units and handset units establish audio links with each other by selectively employing message channels, are known in the prior art. Such telephone apparatus are configured with a control unit connected to a public communication network, a plurality of base units connected to the control unit, and a plurality of handset units which respectively establish audio link with one of the base units. The control unit controls the setting of message channels between the base units and handset units, the registration of the handset unit locations, and others.
A cordless system telephone apparatus of this type covers a radio communication area which is divided into several zones and one or more base units are assigned to each of the zones for the radio communication services with the handset units in the corresponding zones. Furthermore, the same radio frequency band is allocated to each of the zones to avoid radio interferences among them. Namely, radio waves in the same frequency band are used in the multiple zones. This results in simultaneous communications by a number of handset units using radio waves in a narrow frequency band, realizing the efficient utilization of radio waves.
The radio communications between the base units and handset units are conducted on the basis of multi-channel access system, whereby a plurality of message channels are assigned to each of the zones and each of the handset units in one of the zones communicates with the base unit corresponding to the zone through one of the channels.
In such cordless system telephone apparatus, the handset units which can move freely potentially communicate with those base units covering the zones for other handset units. In this case, however, it is difficult for the handset units to exchange radio wave signals of satisfactory qualities with the base units. This raises a necessity for identifying the zones where the handset units communicating with the base units exist, in order to provide the optimum qualities of radio communications.
The base units and handset units communicate with each other through two message channels of different frequencies. In general, one of the two channels is employed for the communications from the handset units to the base units, being referred to as upward message channel and the other for the communications from the base units to the handset units, being referred to as downward message channel. For instance, the frequencies of the two channels are in the 900 MHz band and the difference of the frequencies is 45 MHz for the cordless system telephone apparatus in European countries. In order to identify and register the zones of the handset units, the base units and the handset units of the prior art cordless system telephone apparatus exchange radio wave signals at intervals of a predetermined time during not only audio communications but also waiting states, monitoring the radio wave receiving conditions in control channels or the like. If the magnitudes of the received signals are less than, for instance, 30 dB.mu.V/m, the base units are searched to find out the one for better receiving conditions by switching the base units. Then the zone corresponding to the found out base unit is re-registered for the handset unit in the zone by the control unit.
In this way, the base units and handset units of the conventional telephone apparatus must communicate with each other even during the waiting states, where no audio communications take place, to identify and register the zones for the respective handset units. Therefore, the power of handset unit batteries is additionally consumed, resulting in the relatively shortened lives of the batteries. Also, a pair of upward and downward message channels are occupied for the waiting states in the prior art, conflicting with the efficient utilization of radio waves.